A Q-switched laser produces pulses of light at high peak powers. An intra-cavity shutter is closed, preventing laser action and allowing the energy in the laser gain medium to build up. The shutter is then opened quickly, causing the rapid build-up of the pulse of laser light. Such a shutter is called a Q-switch.
A conventional Q-switched laser system 2 is illustrated in FIG. 1. The laser system 2 includes a resonant cavity formed by two optics: a high reflecting mirror (HR) 4 and an output coupling mirror (OC) 6. A gain medium 8 is placed inside the cavity. When a flashlamp means 9 for exciting the gain medium 8 is activated without a Q-switch device activated, an intracavity laser beam 10 is produced that circulates between the HR 4 and the OC 6. The laser beam 10 is amplified twice during each round trip through the cavity as it traverses the excited gain medium 8. The OC 6 partially transmits a predetermined amount of the intracavity beam 10 to create the output beam 12.
The polarizer 14 linearly polarizes the intracavity beam 10 by offering low optical loss for one (preferred) polarization component and high optical loss for the orthogonal one (non-preferred) polarization component. Under these conditions, beam 10 is forced to build up in the preferred polarization direction only.
Pulsed output is created by intermittently "holding off" the laser. When the losses in the cavity exceed the gain produced by the gain medium 8, lasing in the cavity ceases, and the laser is "held off". When gain exceeds losses, lasing begins again.
To hold off a high gain laser, an electro-optic Q-switch device (EO cell) 16 is placed in the cavity. When high voltage is applied to the EO cell 16 by driver 17 (the cell is on), it acts as a wave plate, thereby having a slow polarization axis and a fast polarization axis. Light polarized in the direction of the slow polarization axis travels slower through the EO cell 16 and becomes phase delayed relative to light polarized in the direction of the fast polarization axis, which travels faster through the EO cell 16. When no voltage is applied to the EO cell 16 (the cell is off), no phase delay occurs. The phase delay induced by an EO cell 16 is proportional to the voltage applied to the EO cell 16.
FIG. 2 illustrates the gradual polarization changes caused by various phase delays induced upon linearly polarized light having a polarization direction 45.degree. to the fast and slow axes of an EO cell 16. As the phase delay is increased from 0.degree., elliptically polarized light is created. The elliptical polarization becomes circularly polarize light when the phase delay is 90.degree.. As the phase delay is increased from 90.degree., elliptically polarized light is created whereby the elongated axis is orthogonal to the elongated axis of the elliptical polarized light created by phase delays of less than 90.degree.. At 180.degree. of phase delay, linearly polarized light is created which is orthogonal to the linearly polarized light at 0.degree. phase delay. Accordingly, a phase delay of 180.degree. results in a 90.degree. rotation of the linear polarization direction.
The phase delay effect of the EO cell 16 is used to rotate the linear polarization direction of the intracavity beam 10 by 90.degree. such that the polarizer 14 rejects the beam. The loss produced by the polarizer 14 is sufficient to hold off the laser. When the EO cell 16 is in its "off" state, there is no polarization rotation, and the laser is no longer held off.
While the laser is held off and the gain medium 8 is being excited (pumped), a large amount of energy is stored in the gain medium 8, since there is no intracavity laser beam to extract such energy. When the Q-switch state is changed quickly, losses drop and lasing begins. The intracavity beam 10 quickly builds and extracts the stored energy thereby producing a very high power light pulse.
In the embodiment shown in FIG. 1, the EO cell 16 is turned off to generate the pulse, and turned on to hold off the laser. There are drawbacks to this type of design. First, an EO cell driver 17 is required that can rapidly switch the high voltage off to turn off the EO cell 16 fast enough to generate the shortest light pulse. It is easier to design a driver 17 that turns the high voltage on quickly, rather than one that turns the high voltage off quickly. Secondly, a high voltage must be applied to the EO cell 16 during the relatively long period of time while the optical gain medium is being pumped. Since EO cells tend to degrade when high voltages are excessively applied, it is not beneficial to have the Q-switch turned on to hold off the laser.
The above mentioned problems are solved by inserting a quarter-wave plate 18 into the cavity, as shown in FIG. 3, where the laser is held off when the EO cell 16 is off, and the light pulse is generated when the EO cell 16 is turned on. During a round trip, the quarter-wave plate 18 rotates the beam polarization by 90.degree., which holds off the laser. When the EO cell is turned on, it either adds an additional 90.degree. polarization rotation, or it adds a polarization rotation equal and opposite to the quarter-wave plate induced rotation, either of which switches the laser on. Therefore, a less complicated driver can be used, and degradation of the EO cell can be avoided (since the EO cell voltage is applied only when the pulse is being generated). The drawback to this cavity configuration is the requirement of an additional intracavity element.
There is a need for a Q-switched laser that does not use an intracavity passive quarter-wave plate whereby the pulse is generated when the EO cell 16 is turned on, and the laser is held off when the EO cell 16 is off.
A drawback to both of the above described embodiments is that the cavity is too long for many applications. Simply shortening the cavity is not a viable solution because certain intracavity dimensions are required to obtain the desired output beam 12. For example, shortening the cavity can increase the divergence of the output beam 12.
A prior art solution to reduce the dimensions of the cavity is a folded cavity configuration, as illustrated in FIG. 4. Two turning mirrors 20 are used to create a "U" shaped cavity, thereby reducing the overall length of the laser head. The use of turning mirrors 20 in the cavity, however, presents several problems. First, intracavity mirrors 20 coated to reflect at 45.degree. can damage easily. The high intracavity powers, especially with Q-switched lasers, can degrade such optical coatings. Further, and more importantly, 45.degree. turning mirrors 20 alter the polarization state of the reflected intracavity beam in a way that is difficult to control. The effect on the polarization state can vary from one coating to the next for the same optical coating design. Therefore, laser cavities sensitive to the polarization of the intracavity beam, such as EO cell Q-switched lasers, cannot use folding mirrors without degradation to laser performance and reliability.
There is a need for a folded laser cavity in which the turning optics do not alter the polarization state of the intracavity beam in an uncontrollable way, and are more reliable than 45.degree. surface coated optics.